docs: def get_mean_and_stddevs(self, sites, rup, dists, imt, stddev_types):
          """
          See :meth:`superclass method
          <.base.GroundShakingIntensityModel.get_mean_and_stddevs>`
          for spec of input and result values.
  
          :raises ValueError:
              if imt is instance of :class:`openquake.hazardlib.imt.SA` with
              unsupported period.
          """
          assert all(stddev_type in self.DEFINED_FOR_STANDARD_DEVIATION_TYPES
                     for stddev_type in stddev_types)
  
          if imt not in self.IMTS_TABLES:
              raise ValueError(
                  'IMT %s not supported in FrankelEtAl1996NSHMP. ' % repr(imt) +
                  'FrankelEtAl1996NSHMP does not allow interpolation for ' +
                  'unsupported periods.'
              )
  
          mean = self._compute_mean(imt, rup.mag, dists.rhypo.copy())
          mean = clip_mean(imt, mean)
  
          stddevs = self._compute_stddevs(imt, dists.rhypo.shape, stddev_types)
  
          return mean, stddevs